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[79.224.245.239]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-451f9919648sm30388695e9.31.2025.06.05.10.04.49 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 05 Jun 2025 10:04:49 -0700 (PDT) Message-ID: <79f75185-ac45-47c6-8f56-2ffa1e4815b0@redhat.com> Date: Thu, 5 Jun 2025 19:04:48 +0200 Precedence: bulk X-Mailing-List: linux-ext4@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 07/12] mm: Remove redundant pXd_devmap calls To: Dan Williams , Jason Gunthorpe Cc: Alistair Popple , linux-mm@kvack.org, gerald.schaefer@linux.ibm.com, willy@infradead.org, linux-kernel@vger.kernel.org, nvdimm@lists.linux.dev, linux-fsdevel@vger.kernel.org, linux-ext4@vger.kernel.org, linux-xfs@vger.kernel.org, jhubbard@nvidia.com, hch@lst.de, zhang.lyra@gmail.com, debug@rivosinc.com, bjorn@kernel.org, balbirs@nvidia.com, lorenzo.stoakes@oracle.com, linux-arm-kernel@lists.infradead.org, loongarch@lists.linux.dev, linuxppc-dev@lists.ozlabs.org, linux-riscv@lists.infradead.org, linux-cxl@vger.kernel.org, dri-devel@lists.freedesktop.org, John@groves.net References: <2ee5a64581d2c78445e5c4180d7eceed085825ca.1748500293.git-series.apopple@nvidia.com> <6841026c50e57_249110022@dwillia2-xfh.jf.intel.com.notmuch> <20250605120909.GA44681@ziepe.ca> <897590f7-99a5-4053-8566-76623b929c7c@redhat.com> <6841c63b3cb25_249110060@dwillia2-xfh.jf.intel.com.notmuch> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 05.06.25 18:30, Dan Williams wrote: > David Hildenbrand wrote: >> On 05.06.25 14:09, Jason Gunthorpe wrote: >>> On Wed, Jun 04, 2025 at 07:35:24PM -0700, Dan Williams wrote: >>> >>>> If all dax pages are special, then vm_normal_page() should never find >>>> them and gup should fail. >>>> >>>> ...oh, but vm_normal_page_p[mu]d() is not used in the gup path, and >>>> 'special' is not set in the pte path. >>> >>> That seems really suboptimal?? Why would pmd and pte be different? >>> >>>> I think for any p[mu]d where p[mu]d_page() is ok to use should never set >>>> 'special', right? >>> >>> There should be dedicated functions for installing pages and PFNs, >>> only the PFN one would set the special bit. >>> >>> And certainly your tests *should* be failing as special entries should >>> never ever be converted to struct page. >> >> Worth reviewing [1] where I clean that up and describe the current >> impact. ;) > > Will do. > >> What's even worse about this pte_devmap()/pmd_devmap()/... shit (sorry! >> but it's absolute shit) is that some pte_mkdev() set the pte special, >> while others ... don't. > > As the person who started the turd rolling into this pile that Alistair > is heroically cleaning up, I approve this characterization. > >> E.g., loongarch >> >> static inline pte_t pte_mkdevmap(pte_t pte) { pte_val(pte) |= >> _PAGE_DEVMAP; return pte; } >> >> I don't even know how it can (could) survive vm_normal_page(). > > Presently "can" because dax switched away from vmf_insert_mixed() to > vmf_insert_page(), "could" in the past was the devmap hack to avoid > treating VM_MIXEDMAP as !vm_normal_page(). The thing is, in vm_normal_page() if we have CONFIG_ARCH_HAS_PTE_SPECIAL -- which loongarch sets -- if we don't see pte_special(), we will assume that it is refcounted. if (likely(!pte_special(pte)) goto check_pfn; So if pte_mkdevmap() does not set pte_special(), then ... vm_normal_page() would detect it as normal, although it isn't normal? But maybe I am missing something important. -- Cheers, David / dhildenb